O₂ Activation over Ag-Decorated CeO₂(111) and TiO₂(110) Surfaces: A Theoretical Comparative\nInvestigation

Abstract

Periodic spin-polarized hybrid density functional theory calculations\nhave been performed to investigate the reactivity of pristine, O-defective,\nand Ag-decorated CeO₂(111) and TiO₂(110) surfaces\nwith a small Ag₁₀ cluster toward O₂. The adsorption\nof O₂ and its subsequent dissociation have been studied\nin order to provide a better understanding of the role of the oxide,\nthe metallic nanoparticle, and their interaction in the reactivity\nof composite metal/metal oxide materials toward O₂, as\npotential catalysts to this reaction. Structural, energetic, electronic,\nand vibrational properties of all species involved in the different\nreaction paths considered have been fully characterized. On the stoichiometric\nsurfaces, Ag₁₀ is oxidized and reduces surface Ce⁴⁺/Ti⁴⁺ ions, while on the O-defective surfaces, the adhesion\nof silver is promoted only on CeO₂ but unfavored on TiO₂. On the other hand, on the silver-free supports, O₂ strongly adsorbs at vacancies and preferentially reduces to peroxide.\nWhen no O vacancies are considered on the Ag₁₀-decorated\nsupports, the net positive charge on Ag₁₀ actually prevents\nthe adsorption and reduction of O₂. Instead, when O vacancies\nare included, two reaction pathways are observed; oxygen molecules\ncan weakly absorb on the silver cluster as a superoxide moiety or\nstrongly adsorb at the vacancy as peroxide. The dissociation of the\nO–O bond of the peroxide is favored both from the thermodynamic\nand kinetic points of view in silver-decorated surfaces, in contrast\nwith the silver-free cases. In addition, Ag₁₀/CeO₂ shows higher activity toward the O₂ adsorption and dissociation\nthan Ag₁₀/TiO₂, which can be related both to\nthe higher ionicity and superior electron storage/release ability\nof ceria with respect to titania, thus leading to the weakening of\nthe O–O bond and providing lower activation barriers for oxygen\nreduction. These results deepen the current understanding of the reactivity\nof metal/metal oxide composites toward O₂, especially elucidating\nhow the surface stoichiometry affects the charge state of the metal\nclusters, and hence the reactivity of these interfaces toward O₂, with potential important consequences when such composites\nare considered for catalytic applications.